The present invention relates to a two-dimensional photonic crystal device and, more particularly, to a two-dimensional photonic crystal device for use in WDM (Wavelength Division Multiplex) communications and in similar technical fields.
Over the past few years, spectacular technological advances have been made in the field of optical wavelength division multiplexing (hereinafter referred to as WDM) communications. These technological progresses aim to meet the demands by network users for higher communication speeds in the Internet. The WDM technology is now going mainstream in the construction of optical communication networks although the use of expensive WDM components inevitably raises the overall cost of optical communication systems in the Internet. Nowadays, optics is used in various fields of transmission of information through optical communication networks. The next generation optics is expected as technologies that implement some or all of switching and control operations in such optical networks. The new optics technologies will lead to realization of low-cost, high-performance WDM components.
Incidentally, the WDM technology is based on a variety of conventional technologies, such as Fabry-Perot filter, thin film interference filters, Mach-Zehnder filters, birefringent filters, fiber-gating Bragg reflection Filters and waveguide diffraction gratings. On the other hand, a low-loss, bend waveguide in optical ICs needs to be gently bent over as long a distance as hundreds of μm—this inevitably makes the optical ICs bulky.
A multi-channel WDM device smaller in size than conventional devices can be obtained through utilization of photonic crystal technologies. The photonic crystals are artificial periodic structures with two kinds of transparent media of greatly different refractive indexes alternately arranged at regular intervals of the order of half-wavelength of light. With photonic crystals, it is possible to fabricate a one-piece, high-density compound device that is handled on the scale of the order of sub-millimeters and bends light in a predetermined direction and hence exhibits multiple optical functions.
Such photonic crystals fall roughly into a one-dimensional photonic crystal formed by a one-dimensional periodic alternation of media of refractive indexes n1 and n2, a two-dimensional photonic crystal formed by a two-dimensional periodic alternation of media of refractive indexes n1 and n2, and a three-dimensional photonic crystal formed by a three-dimensional periodic alternation of media of refractive indexes n1 and n2. Such photonic crystals use silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or similar semiconductor as the medium of the refractive index n1 which is, for instance, the higher refractive index, and air or glass as the medium of the low refractive index n2.
The photonic crystals have a periodic structure with the elements arranged at intervals of the order of half-wavelength of light as described above. For example, when the photonic crystal of the two-dimensional periodic structure uses air as the medium of the low refractive index and a dielectric as the medium of the high refractive index, the elements of the periodic structure may be dielectric pillars arranged in the air or cavities arranged in the dielectric. One of unique features of the photonic crystal is a photonic band gap, which is a frequency range in which light of a certain range of wavelengths is prevented from propagation. The photonic crystal can be used for control of light since an optical waveguide can be formed in the crystal by introducing thereinto crystal defects. The waveguide in the photonic crystal is provided by an omission of one line of elements of the periodic structure.
In Japanese Patent Application Kokai Publication No. 2001-272555 gazette (published Oct. 5, 2001, hereinafter referred to as document 1) there is set forth an add-drop filter of the type having crystal defects formed along the waveguide. FIG. 1 depicts the add-drop filter described in document 1, which has a two-dimensional photonic crystal structure in which a slab 11, formed of a material higher in refractive index than air, has columnar holes 16 perforated therein in a triangular lattice pattern to form a refractive index distribution. In the periodic structure of the photonic crystal there is provided crystal defects which form a line waveguide 12, and the diameter of one of cavities adjacent the waveguide is changed to form a point defect 14 which disturbs the periodic arrangement of elements of the photonic crystal. Of light 13 in the wavelength range including a plurality of wavelengths (λ1, λ2, . . . λi, . . . ) that are launched into the waveguide for propagation therethrough, light 15 of a particular wavelength (λi) is captured by the point defect 14 and emitted in a direction perpendicular to the top surface of the slab 11.
In C. Jin, S. Han, X. Meng, B. Cheng, and D. Zhang, “Demultiplexer using directly resonant tunneling between point defects and waveguides in a photonic crystal,” J. Appl. Phys., Vol. 91, No. 7, P. 4771-4773, 1 Apr. 2002 (hereinafter referred to as document 2) there is set forth such a two-dimensional photonic crystal device as shown in FIG. 2A. In FIG. 2A, in the periodic structure of the two-dimensional photonic crystal, point defects C1, C2, C3 and C4 are formed by pillars different in diameter from pillars 71 of the periodic structure and of different refractive indexes are provided. A line waveguide 73, which is formed by an omission of some lines of the pillars 71, extends from one side 72 of the two-dimensional photonic crystal structure to the other side 74 thereof. The waveguide 73 is branched into four waveguides one after another which extend to the other side 74 passing by the point defects C1, C2, C3 and C4, respectively. And the pillars 71 of the periodic structure are located in pairs at the front and rear of each of the point defects C1, C2, C3 and C4 in the lengthwise direction of the respective waveguide. Light of a wide band launched into the waveguide 73 from the one side 72 is transmitted through the point defects C1, C2, C3 and C4 acting as resonators, as shown in FIG. 2B.
In S. Fan, P. R. Villeneuve, J. D. Joannopoulos, and H. A. Haus, “Channel drop filters in photonic crystals,” Opt. Express, Vol. 3, No 1, P. 4-11, 6 Jul. 1998 (hereinafter referred to as document 3), too, there is set forth such a two-dimensional photonic crystal device as shown in FIG. 3. In the two-dimensional photoic crystal structure, two line waveguides 81 and 82 are provided, and in the periodic structure defined by the two waveguides 81 and 82 a point defect 84 is provided which differs in diameter and in refractive index from pillars 83 of the periodic structure. The point defect 84 is used as a resonator. Of wavelength multiplexed light that propagates through the waveguide 81, for instance, light of a particular wavelength alone is separate by the point defect 84 for propagation through the waveguide 82.
Conventionally, as described above, the two-dimensional photonic crystal device as a wavelength selective filter has a structure in which a line waveguide is formed by crystal defects in the photonic crystal structure and a point defect as a resonator is formed adjacent the waveguide in the periodic structure of the photonic crystal.
However, in the two-dimensional photonic crystal device of document 1, for example, when the wavelength of light to be propagated is 1.55 μm, the thickness of the wall of the slab 11 between the cavities 16 of the periodic structure and the cavity forming the point defect 14 is as thin as around 0.06 μm; therefore, it is difficult to fabricate the device of document 1. In the devices of documents 2 and 3, it is necessary that a pillar different in refractive index from the pillars of the periodic structure of the two-dimensional photonic crystal be formed as a point defect in the periodic structure, but changing the refractive index is not easy from the viewpoint of fabrication. In particular, the device of document 2, which has four point defects and four channels, requires, in addition to two media forming the photonic crystal, four media different in refractive index from them, hence the device is extremely difficult to fabricate.